1. Field of the Invention
The present invention relates generally to semiconductor fabrication. More particularly, the present invention involves detecting trapped charge densities in films, such as dielectric films.
2. Description of Related Art
As device sizes continue to be reduced and a there is a demand for higher performance and faster products, scaling of the traditional gate oxide layer has gone below 2 nanometers. As a result, leakage currents due to tunneling have increased, thereby inhibiting the performance of devices. Current integrated circuit (IC) manufacturing processes have implemented the replacement of traditional gate oxide layer, generally silicon dioxide (SiO2) with high-k dielectrics to maintain a low gate leakage current density.
However, poor transistor drive current due to degraded field effect carrier mobility is a major issue with high-k dielectrics. This affect counters one of the top priorities of successful IC scaling goals which include an improvement of mobility. The observed degradation of mobility has been primarily attributed to localized charge traps at the interface between a crystalline substrate and a dielectric material. For example, Hafnium based dielectrics have been studied extensively, both theoretically and experimentally, to determine both the density of these defects and the energy at which they lie within the band gap of the dielectric. It has been found that for HfO2 with a band gap of approximately 6.0 electron volts (eV), the charge traps are oxygen vacancies and interstitials that reside about 0.3-0.5 eV below the band gap and have a density of about 1×1011 to about 1×1013 cm−2.
Much effort has been put forth to decrease the amount of these oxygen vacancies by introducing other elements into the film (i.e. hafnium silicates) or with process optimization (e.g., high temperature anneals which fill the vacancies with the anneal ambient). However, during the manufacturing process, there is no method available to determine the extent of charge trapping. It is therefore necessary for an in-line metrology technique to be available that can detect gate dielectrics with a high density of charge traps before transistor processing is complete.
Unfortunately, current techniques to determine the extent of charge traps introduced into the dielectric film include electric capacitance-voltage (CV) measurements and current-voltage (IV) measurements, which are offline techniques. Observation of hysteresis in the capacitance versus the voltage curves may indicate charge trapping. This technique is an off-line, destructive technique (e.g., requiring sacrificial wafers for processing), and in many cases (especially high-k development), has failed to reflect the reality of the complex trapping structures in the dielectrics due to subsequent processing conditions such as high temperature anneal, gate electrode metal deposition, and the like.
Another technique, modulation spectroscopy, which includes photo reflectance, electro reflectance, piezo reflectance, thermo reflectance, and the like, may be used to characterize charge trapping. Generally, a reflectivity induced by a pertubative mechanism (e.g., laser, electric field, stress, temperature, and the like) applied to the material is measured. In particular, modulation spectroscopy measures the normalized change in reflectivity described by
                              Δ          ⁢                                          ⁢          R                R            =                        α          ⁢                                          ⁢          Δ          ⁢                                          ⁢                      ɛ            1                          +                  β          ⁢                                          ⁢          Δ          ⁢                                          ⁢                      ɛ            2                                ,                  ⁢    where        α    =                                                      ∂              L                        ⁢                                                  ⁢            n            ⁢                                        R                                                          ∂                          ɛ              1                                      ⁢                                  ⁢        and        ⁢                                  ⁢        β            =                                                  ∂              L                        ⁢                                                  ⁢            n            ⁢                                        R                                                          ∂                          ɛ              2                                      .            The modulation spectroscopy techniques provides some benefits by accentuating the discontinuities in the real (∈1) and imaginary (∈2) part of the dielectric function at the various critical points in the band structure. These features are enhanced by measuring the differential of the critical points (Δ∈), while baseline features associated with the film thickness, roughness, and other 2nd order effects are suppressed.
However, modulation techniques are unable to accurately measure the charge trap induced changes in the substrate dielectric function because of the functional dependents of Seraphin coefficients (α and β) that act to scale the changes in the real or imaginary part of the dielectric function. For example, for a multilayer system (e.g., gate dielectric layer and substrate layer), the reflectivity R, is a function of film thickness, dielectric function of each film, and incident angle. Therefore, the Seraphin coefficient that multiplies the change in the dielectric function will be different for different films and thicknesses and would affect the amplitude of the change in the dielectric function for different thicknesses, compositions, etc. Thus, it would be difficult to extract information about the charge trap induced electric field without knowing exactly the thickness of the film, dielectric function of the film, and incident angle of each layer because modulations techniques do not provide thickness of films, dielectric function of film, and incident angles.
Any shortcoming mentioned above is not intended to be exhaustive, but rather is among many that tends to impair the effectiveness of previously known techniques for identifying charge traps in dielectric material; however, shortcomings mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.